This invention relates generally to gas turbine combustors and more particularly concerns reducing combustion instabilities in dry low NO.sub.x gas turbine combustors.
Gas turbines generally include a compressor, one or more combustors, a fuel injection system and a turbine. Typically, the compressor pressurizes inlet air which is then reverse flowed to the combustors where it is used to provide air for the combustion process and also to cool the combustors. In a multi-combustor system, the combustors are located about the periphery of the gas turbine, and a transition duct connects the outlet end of each combustor with the inlet end of the turbine to deliver the hot products of combustion to the turbine.
Gas turbine combustors are being developed which employ lean premixed combustion to reduce emissions of gases such as NO.sub.x. One such combustor comprises a plurality of premixers attached to a single combustion chamber. Each premixer includes a flow tube with a centrally-disposed fuel nozzle comprising a center hub which supports fuel injectors and swirl vanes. During operation, fuel is injected through the fuel injectors and mixes with the swirling air in the flow tube, and a flame is produced at the exit of the flow tube. The combustion flame is stabilized by a combination of bluffbody recirculation behind the center hub and swirl-induced recirculation. Because of the lean stoichiometry, lean premixed combustion achieves lower flame temperatures and thus produces lower NO.sub.x emissions.
Because of the turbulent nature of the combustion process and the large volumetric energy release in closed cavities, such combustors are susceptible to a wide range of modes and frequencies of combustion-induced unsteady pressure oscillations of large amplitudes. These pressure oscillations, referred to herein as "dynamics," can severely limit the combustor operating range and can even destroy combustor hardware. Methods to suppress combustor dynamics have traditionally worked upon de-coupling the excitation source from the feedback mechanism. Such means are generally only effective over a limited range of operation of the combustor.
Accordingly, there is a need for a low NO.sub.x combustor capable of achieving low dynamics over a wide range of operation.